The present invention is in the field of underground pipelines and is more specifically directed to a pipeline installation in areas in which the pipeline extends through adjacent soil zones having different frost heave driving forces and different resistive forces opposing the upward frost heave forces. The invention is of particular value in arctic and sub-arctic areas in which permafrost frozen soil conditions exist on a year-round basis at varying depths beneath the soil surface. However, it should also be understood that use of the present invention is not limited to permafrost areas and the benefits of the invention are achievable in any installation in which a chilled gas pipeline traverses alternate zones having different freeze and heave characteristics. Thus, the present invention relates to a method and structure for reducing forces exerted on a buried chilled gas pipeline extending through permafrost or other zones having different freezing and heaving characteristics which could create excessive force on the pipeline causing a hazardous likelihood of damage or rupture thereof.
A better understanding of the problems to which the present invention is addressed will be achieved by reference to FIG. 1 of the drawings which illustrates a pipeline P extending through a soil zone B having substantial frost heave and adjacent soil zones A and C having less frost heave; pipeline P is consequently subjected to differential heaving forces which, if of sufficient magnitude, could rupture the pipeline. Soil zone B having substantial frost heave attempts to push the pipe upward through the lesser heaving adjacent soil zones A and C. Resistance to the upward movement by the lesser heaving soil zones A and C is referred to as uplift resistance and it is the forces generated by the oppositely acting frost heave driving forces in zone B and uplift resistance forces in zones A and C which can create a hazardous likelihood of pipe failure.
The foregoing problems are most acute in arctic regions where a mixture of soil, rock, and ice, which is referred to as permafrost, remains in essentially permanently frozen condition downwardly from a depth a few feet below or near the surface. The surface soil layer above the permafrost layer is subjected to alternate thawing and freezing during the warm and cold seasons. However, discontinuous permafrost areas occur in which a thawed "active" area will be positioned between permanently frozen areas and will extend downwardly to bedrock or to a thaw line at a greater depth than the surface thawed portions of adjacent surface soil layers. The problems of maintaining structural integrity and stability by reducing the strain resultant from the differential heave forces are particularly acute for pipelines in such circumstances. A chilled gas pipeline carrying gas at below freezing temperatures is susceptible to frost heave forces since a frost bulb buildup around the pipe will in some soils attract additional moisture so as to increase the differential frost heave forces exerted on the pipe.
Soviet Pat. No. 361,349 discloses a pipeline having insulation about the lower half of the pipe apparently for the purpose of reducing pipe stress by reducing the growth of the frost bulb below the pipe and hence the frost heave forces. It would appear that the pipe disclosed in this patent is a liquid pipeline. German Pat. No. 497,118 also discloses a pipeline having varying amounts of insulation about different surfaces.
Devices, which have been somewhat misleadingly referred to as "heat pipes", such as exemplified in U.S. Pat. No. 3,217,791, have comprised a sealed pipe having a quantity of low boiling point liquid on their interiors. Such pipes have been embedded in the soil with their upper ends extending into the atmosphere. In such devices, the transfer of the heat to the colder atmosphere is effected by the change of state of the low boiling point liquid provided in the bottom of the pipe which absorbs heat from the surrounding soil and evaporates so that vapors move to the top of the pipe wherein the vapors are cooled by the surrounding cooler atmosphere and condensed to flow back to the bottom of the pipe in a continuous cycle of operation.
U.S. Pat. Nos. 4,194,856 and 4,269,539 disclose the employment of heat pipes positioned either adjacent to or beneath a refrigerated gas pipeline for aiding in the maintaining of a frozen condition beneath the pipeline so as to avoid the creation of excessive forces on the pipeline. These patents also include an extensive prior art discussion to which attention is particularly invited. Other known prior art includes U.S. Pat. Nos. 3,563,825; 3,747,355; 3,807,183; 3,809,149; 3,948,313 and 3,990,502.
Unfortunately, the prior known systems for controlling frost heave of pipelines have been unsatisfactory in performance and/or have been extremely expensive to manufacture and/or maintain.
Therefore, it is the primary object of the present invention to provide a new and improved method and apparatus for avoiding frost heave damage to a pipeline.
An even more particular object of the invention is the provision of a new and improved apparatus and method for preventing frost heave damage to chilled gas pipelines.
A still further object of the present invention is the provision of a new and improved apparatus and method for preventing damage to chilled-gas pipelines passing across different soil zones having different freezing and frost heave characteristics.